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United States Patent |
5,517,198
|
McEwan
|
May 14, 1996
|
Ultra-wideband directional sampler
Abstract
The Ultra-Wideband (UWB) Directional Sampler is a four port device that
combines the function of a directional coupler with a high speed sampler.
Two of the four ports operate at a high sub-nanosecond speed, in "real
time", and the other two ports operate at a slow millisecond-speed, in
"equivalent time". A signal flowing inbound to either of the high speed
ports is sampled and coupled, in equivalent time, to the adjacent
equivalent time port while being isolated from the opposite equivalent
time port. A primary application is for a time domain reflectometry (TDR)
situation where the reflected pulse returns while the outbound pulse is
still being transmitted, such as when the reflecting discontinuity is very
close to the TDR apparatus.
Inventors:
|
McEwan; Thomas E. (Livermore, CA)
|
Assignee:
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The Regents of the University of California (Oakland, CA)
|
Appl. No.:
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510956 |
Filed:
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August 3, 1995 |
Current U.S. Class: |
342/89; 73/290R; 375/141; 375/145; 375/146; 375/149 |
Intern'l Class: |
G01S 013/00 |
Field of Search: |
375/200
342/89
|
References Cited
U.S. Patent Documents
5220683 | Jun., 1993 | Rudish | 455/146.
|
5345471 | Sep., 1994 | McEwan | 375/1.
|
5455593 | Oct., 1995 | Ross | 342/375.
|
5465094 | Nov., 1995 | McEwan | 342/28.
|
Primary Examiner: Eldred; J. Woodrow
Attorney, Agent or Firm: Sartorio; Henry P.
Goverment Interests
The United States Government has rights in this invention pursuant to
Contract No. W-7405-ENG-48 between the United States Department of Energy
and the University of California for the operation of Lawrence Livermore
National Laboratory.
Parent Case Text
RELATED APPLICATIONS
This application is a continuation-in-part (CIP) of application Ser. No.
08/301,924 filed Sep. 6, 1994pending, which is a continuation-in-part
(CIP) of application Ser. No. 08/044,745 filed Apr. 12, 1993, now U.S.
Pat. NO. 5,345,471, which are herein incorporated by reference.
Claims
The Invention Claimed is:
1. An ultra-wideband (UWB) directional sampler, comprising:
an electronic coupler having a first port operating in real time, a second
port operating in real time and coupled to the first port, a third port
coupled to the first port and isolated from the second port and providing
an equivalent time representation of a signal at the first port, and a
fourth port coupled to the second port and providing an equivalent time
representation of a signal at the second port;
a sampling gate pulse generator for sampling signals at the first and
second ports.
2. The UWB directional sampler of claim 1 wherein the electronic coupler
comprises a resistor bridge circuit connected between the first and second
ports, a differential sampler having first and second channels connected
from the bridge circuit to the fourth port, and an inverting amplifier
connected from the first channel to the third port.
3. The UWB directional sampler of claim 2 further comprising a pair of
diodes connected together between the first and second channels and
connected to the sampling gate pulse generator.
4. The UWB directional sampler of claim 3 wherein each channel comprises a
charge holding capacitor in series with an isolation resistor connected to
an input of a differencing amplifier, the output of the differencing
amplifier being connected back to a second input.
5. The UWB directional sampler of claim 3 further comprising a third
differencing amplifier having an input connected to the output of the
differencing amplifier in the first and second channels through a
differentiating capacitor and series resistor.
6. Apparatus for measuring the level of a material, comprising:
a pulse generator for generating a real time transmit pulse;
a directional sampler having a first port for receiving the real time
transmit pulse, a second port coupled to the first port for transmitting
the real time transmit pulse, a third port coupled to the first port and
isolated from the second port, and a fourth port coupled to the second
port;
a dipstick assembly connected to the second port;
first and second comparators connected to the third and fourth ports;
a set-reset flip-flop connected to the first and second comparators.
7. The apparatus of claim 6 wherein the third and fourth ports of the
directional sampler are equivalent time ports.
8. The apparatus of claim 6 wherein the dipstick assembly comprises a guide
wire for partial immersion in the material, and a launcher plate at the
beginning of the guide wire.
9. The apparatus of claim 6 further comprising an interconnect cable
between the second port and the dipstick assembly.
10. The apparatus of claim 6 further comprising a range counter connected
to the output of the flip-flop.
Description
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to directional couplers and high speed samplers.
Copending patent application Ser. No. 08/359,090 filed Dec. 19, 1994
describes an electronic material level sensor or "electronic dipstick"
based on time domain reflectometry (TDR) of very short electrical pulses.
Pulses are propagated along a transmission line or guide wire that is
partially immersed in the material being measured. A launcher plate is
positioned at the beginning of the guide wire. Reflected pulses are
produced at the material interface due to the change in dielectric
constant. The time difference of the reflections at the start of the guide
wire and at the material interface are used to determine the material
level.
However, there is a problem when the reflected pulse returns while the
outbound pulse is still being transmitted, i.e. when the reflecting
discontinuity is very close to the input or launch end of the transmission
line. In these cases it may be necessary to connect the electronics to the
transmission line through a cable to sufficiently separate the transmitted
and reflected pulses. However, there is a need for a system which does not
require a cable wherein the electronics can be connected directly to the
launch point of the dipstick.
U.S. Pat. No. 5,345,471 and copending CIP application Ser. No. 08/301,924
filed Sep. 6, 1994 describe an ultra-wideband (UWB) receiver which
utilizes a strobed input line with a sampler connected to an amplifier.
The outputs of two integrating single-ended samplers are input into a
differencing amplifier. The samplers integrate, or average, up to 10,000
pulses.
SUMMARY OF THE INVENTION
Accordingly it is an object of the invention to provide method and
apparatus to couple the electronics to a transmission line in a TDR system
so that the reflected pulses are separated from the transmitted pulses.
It is another object of the invention to provide an ultra-wideband
directional sampler which combines the functions of a directional coupler
with a high speed sampler.
The Ultra-Wideband (UWB) Directional Sampler of the invention is a four
port device that combines the function of a directional coupler with a
high speed sampler. Two of the four ports operate at a high sub-nanosecond
speed, in "real time", and the other two ports operate at a slow
milli-secondspeed, in "equivalent time". A signal flowing inbound to
either of the high speed ports is sampled and coupled, in equivalent time,
to the adjacent equivalent time port while being isolated from the
opposite equivalent time port.
A primary application is for a time domain reflectometry (TDR) situation
where the reflected pulse returns while the outbound pulse is still being
transmitted, such as when the reflecting discontinuity is very close to
the TDR apparatus. In commercial applications of TDR, such as an
electronic dipstick for automotive gas tanks, this situation arises when
the TDR pulse is launched onto a rod from the top of the tank and the tank
is full. In this case, the pulse is reflected back from the gas to the TDR
electronics while it is still being transmitted, causing the sampler to
sample the transmitted pulse along with the reflected pulse, resulting in
a substantial error. The directional sampler isolates the transmitted
pulse from the reflected pulse so an accurate level measurement can be
made on just the reflected pulse.
The directional sampler is also useful in suppressing close-in clutter in
UWB radar systems where the directional sampler is used as a receiver. The
directional sampler cancels the transmitted pulse, thereby reducing a
major source of radar clutter at close ranges.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of the differential sampler.
FIG. 2 is a block diagram of a differential sampler connected to a
transmission line.
FIG. 3 is a circuit diagram of a differential sampler connected to a
transmission line.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 depicts the general arrangement of the directional sampler 10. There
are four ports where ports 1 and 2 are "real time" ports and ports 3 and 4
are sampled "equivalent time" ports. Ports 1 and 2 are bidirectional
regarding signal flow. By convention port 1 is the input port that couples
a transmitted signal (T) to port 2. Port 2 is normally connected to an
antenna or a transmission line used for TDR purposes. A GATE signal is
applied to directional sampler 10 to operate the sampler.
Ports 3 and 4 are output ports from a differential sampler within the
coupler, and are not bidirectional. They are outputs only and the outputs
are equivalent time replicas of the real time signals at ports 1 and 2.
The signal flow in FIG. 1 shows a transmit input T applied to port 1 that
is coupled to port 2 in real time and coupled to port 3 in equivalent
time. That is, an equivalent time replica of T appears at port 3.
Generally, the real time T input is a 200 picosecond wide pulse that also
appears at port 2, and a replica of T appears at port 3 with a pulse width
of 200 microseconds.
A portion of the signal T exiting port 2 reflects back as a reflected
signal R into port 2. This signal is sampled and appears in equivalent
time at port 4 but not at port 3.
The equivalent time T and R signals are used to gate a range counter, as
shown in FIG. 2 where pulser 12 is connected to input port 1 of
differential sampler 12 through a transmission line 14. Port 2 of sampler
10 is connected through interconnect cable 16 to transmission line or
guide wire ("stick") 20. A launcher plate 18 is mounted at the junction
between lines 16 and 20. Transmission line 20 extends into a liquid (or
other material) 22. The equivalent time T and R signals at ports 3 and 4
are connected to threshold comparators 24 and 26 that have their reference
inputs connected to voltages -V.sub.REF and +V.sub.REF that are about half
the peak amplitudes of the T and R signals, respectively. The comparator
24, 26 outputs drive a flip-flop 28 to generate a variable width range
pulse that gates a range counter 30.
The equivalent time range scale is typically 1 ms=1 inch. If a 1 MHz
counter 30 is gated by the range pulse, each count at a 1 microsecond
interval corresponds to 0.001". Thus, 0.001" is the level of precision at
which the level of fluid 22 can be measured.
Also depicted in FIG. 2 is a typical real time connection of the
directional sampler 10, showing illustrative pulses at various points in
the circuit, including a 0.2ns pulse generator 12 providing signal T to
port 1 and showing T exiting port 2 to a dipstick assembly 32. The
dipstick assembly 32 comprises a launcher plate 18 and a metallic guide
wire or "stick" 20. The operation of dipstick assembly 32 is described in
U.S. patent application Ser. No. 08/359,090 which is herein incorporated
by reference. Pulses are reflected from the liquid 22 that the stick 20 is
inserted into and provide a reflected signal R back into port 2. An
equivalent time replica of R appears at port 4 while an equivalent time
replica of T appears at port 3.
In operation, when a signal T of sufficient magnitude, i.e. exceeding
-.sub.VREF, is input into sampler 10 through port 1, and thereby launched
into guidewire 20 through port 2, flip-flop 28 is set. When a reflected
signal of sufficient magnitude, i.e. exceeding +V.sub.REF, appears at port
4, flip-flop 28 is reset. Since T can never reach port 4, and R can never
reach port 3, the transmit signal T can never reset flip-flop 28 and the
reflected signal R can never set flip-flop 28. Thus, even if T and R are
very close in time and even overlapping, e.g. when the level of liquid 22
is very near launch plate 18, T and R can only operate on the flip-flop 28
so that the time difference in the reflected signal is accurately
measured, giving precise fluid level measurements.
FIG. 3 provides a schematic diagram of the directional sampler 10. Input
port 1 is terminated by resistor R.sub.T1 to ground and is further
connected to the top of a bridge circuit 34 comprised of resistors
R.sub.1, R.sub.2, R.sub.3, R.sub.4. Thus input pulse T at port 1 is split
by resistors R.sub.1, R.sub.2 into two channels 38, 40 of differential
sampler 10. Port 2 is connected to the right side of the bridge and
receives the port 1 signal T through R.sub.2. R.sub.4, in combination with
R.sub.2, provides a termination for port 2 that matches the impedance
Z.sub.o of the transmission line 16 connected to port 2.
The left side of the bridge is formed by R.sub.1 and R.sub.3, and is
terminated by R.sub.T2 =Z.sub.o. The left and right sides of the bridge
are connected to a differential UWB sampler 10 as described in U.S. Pat.
No. 5,345,471 and copending CIP application Ser. No. 08/301,924 which are
herein incorporated by reference.
A pair of input terminals, one connected to port 2, the other connected to
port 1 through R.sub.1, are respectively connected through a series
capacitor and resistor C.sub.H -R.sub.ISO to the positive inputs of a pair
of differencing (operational) amplifiers A1, A2. The input channels are
commonly strobed (GATE) through diodes D.sub.1, D.sub.2. The strobe line
may include a capacitor CG for pulse shaping, if required, and is
terminated in a resistor R.sub.G. The capacitors C.sub.H act as charge
holding capacitor's. The resistors R.sub.ISO isolate the strobe pulse from
appearing at the amplifier input terminals, and from being shunted to
ground by C.sub.F1, C.sub.F2. A parallel resistor and capacitor R.sub.2
--C.sub.F1, R.sub.3 --C.sub.F2 is also connected from each amplifier
positive input to ground and act as integrators for voltage developed
across C.sub.H. The outputs of amplifiers A1, A2 are connected back to the
negative inputs of A1, A2. The output of A1, since it is connected to port
1, is the (negative) transmit signal T. The output of A2, since it is
connected to port 2, is the (negative) sum of both the transmit signal T
and the reflected signal R. The values of the resistances R.sub.1 -R.sub.4
in the bridge 34 are chosen so that R cannot couple back to A1.
The outputs of amplifiers A1, A2 are input through series capacitor C.sub.i
and resistor R.sub.i into the positive and negative inputs, respectively,
of differencing amplifier A4, whose output at port 4 is R. The negative
input of A4 is also connected to its output through parallel R.sub.F
-C.sub.F4B while the positive input of A4 is connected to ground through
parallel R.sub.F -C.sub.F4A. The output of A1 is input into an inverting
amplifier A3 whose output at port 3 is T. The output of A1 passes through
series C.sub.i, R.sub.i to the negative input of A3 whose positive input
is connected to ground. The output of A3 is also connected back to its
negative input through parallel C.sub.F3 -R.sub.F.
Capacitors CH form signal integrators, i.e., the time constant R.sub.T2
.multidot.C.sub.H is substantially larger than the sampling gate width
(GATE) provided by the pulse generator 36, and the discharge time of
C.sub.H is larger than the pulse repetition interval (PRI) of the pulse
generator. The discharge time of C.sub.H is set by the current provided
through R.sub.2, R.sub.3 connected from the positive inputs of amplifiers
A1, A2. R.sub.2, R.sub.3 have typical values in the megohm range and
C.sub.H may typically be 0.01 microfarad, for pulse generator pulse widths
of 0.1ns and a PRI of 1 microsecond.
Resistors R.sub.ISO isolate the UWB frequencies from amplifiers A1, A2.
Their value must be high compared to R.sub.T2, but not so high that noise
performance is degraded. A typical value is 10K ohms. The product of
R.sub.ISO times the input capacitance of A1, A2 must be high compared to
the pulse generator pulse width.
Additionally, capacitors C.sub.F1, C.sub.F2 may be placed from the plus
inputs of A1, A2 to ground to improve isolation, and to prevent radio
frequencies from the UWB inputs from appearing at the amplifiers inputs,
which could result in erratic performance. Only detected baseband voltages
should appear at the amplifier inputs.
Amplifiers A1, A2, A4 form a standard high input impedance fully
differential amplifier. In this embodiment, A1, A2, and A4 are J-FET input
operational amplifiers contained on a single chip, part number TLO-74 by
Texas Instruments. C.sub.i -R.sub.i and C.sub.FX -R.sub.FX (where X=3, 4A,
or 4B) function as differentiators and integrators, respectively, or in
combination, as a baseband bandpass filter.
A pair of diodes D1, D2 are connected in series, cathode to cathode, from
the junctions between the pairs of C.sub.H and R.sub.ISO. Pulse generator
36 is connected to the junction between D1 and D2 (through optional
C.sub.G).
By symmetry, input signal T will appear equally at both inputs (ports 1, 2)
to the differential sampler 10. The differential sampler employs
operational amplifiers Ai, A2, and A4 to perform a subtraction between its
two inputs, and thus T does not appear at port 4 output. However, op amps
A1 and A3 are connected in a single ended configuration to the left side
of the bridge only and provide a sampled output of signal T to port 3.
Reflected signal R appears at the right side of the bridge and in greatly
attenuated form at the left side of the bridge. The right side of the
differential sampler samples R and applies the result to port 4. Thus only
R and not T appears at port 4.
Signal R is attenuated by two successive dividers R2, R.sub.T1, and then by
R.sub.1, R.sub.T2 before it reaches the left side of the differential
sampler. Thus only a very small portion of R appears as leakage at port 3,
so the port 3 output substantially contains only signal T.
Typically R.sub.1 =R.sub.2 =560 ohms, R.sub.3 =R.sub.4 =56 ohms, Z.sub.o
=R.sub.T2 =50 ohms and R.sub.T1 =82 ohms. With these values, the leakage
of R into port 3 is about -40dB. The leakage of R can be perfectly
canceled if a matching, attenuated portion of A2's output is applied to
the plus input of A3. Of course anyone skilled in the art can fine tune
these values to achieve a precise match with Z.sub.o. It should also be
understood that R.sub.1 -R.sub.4 may also be reactances formed by
inductors or capacitors.
The gate pulse (GATE) is typically a +5 to 0 volt transition from a logic
gate or transistor. The transition is differentiated into a pulse by
R.sub.G, C.sub.G and applied to the cathodes of Schottky diodes D.sub.1,
D.sub.2, typically Hewleft Packard type HSMS2814. The gate pulse recurs
with a typical repetition rate of 2MHz.
With the invention, because ports 3 and 4 of the directional sampler are
isolated from each other, and the reflected signal from the material
cannot be confused with the transmit signal, the inteconnect cable from
port 2 to the dipstick assembly can be eliminated so that the electronics
can be connected directly to the dipstickassembly.
Changes and modifications in the specifically described embodiments can be
carried out without departing from the scope of the invention which is
intended to be limited only by the scope of the appended claims.
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